Problem: In chemistry class, the girl to boy ratio is $4$ to $9$. If there are a total of $39$ students, how many girls are there?
Answer: A ratio of $4$ girls to $9$ boys means that a set of $13$ students will have $4$ girls and $9$ boys. A class of $39$ students has $3$ sets of $13$ students. Because we know that there are $4$ girls in each set of $13$ students, the class must have $3$ groups of $4$ girls each. There is a total of $12$ girls in chemistry class.